Geoscience Reference
In-Depth Information
Precipitation (cm)
< 40
40-80
80-120
120-160
160+
Figure 2.1
Annual precipitation across the USA during 1996.
Source
: Redrawn with data from the National Atmospheric Deposition Program
of static and dynamic factors. In Figure 2.1 the
rainfall distribution across the USA shows marked
variations. Although mountainous areas have a
higher rainfall, and also act as a block to rainfall
reaching the drier centre of the country, they do not
provide the only explanation for the variations
evident in Figure 2.1. The higher rainfall in the
north-west states (Oregon and Washington) is
linked to wetter cyclonic weather systems from the
northern Pacific that do not reach down to southern
California. Higher rainfall in Florida and other
southern states is linked to the warm waters of the
Caribbean sea. These are examples of dynamic
influences as they vary between rainfall events.
At smaller scales the static factors are often more
dominant, although it is not uncommon for quite
large variations in rainfall across a small area caused
by individual storm clouds to exist. As an example:
on 3 July 2000 an intense rainfall event caused
flooding in the village of Epping Green, Essex, UK.
Approximately 10 mm of rain fell within one hour,
although there was no recorded rainfall in the village
of Theydon Bois approximately 10 km to the south.
This large spatial difference in rainfall was caused
by the scale of the weather system causing the storm
- in this case a convective thunderstorm. Often
these types of variation lessen in importance over
a longer timescale so that the annual rainfall in
Epping Green and Theydon Bois is very similar,
whereas the daily rainfall may differ considerably.
For the hydrologist, who is interested in rainfall at
the small scale, the only way to try and characterise
these dynamic variations is through having as many
rain gauges
as possible within a study area.
Static influences on precipitation
distribution
It is easier for the hydrologist to account for static
variables such as those discussed below.
